The company has traced the problem to a malfunction in the instruments that monitor the speed of the turbine, said Jill Lyon, speaking for the company. The malfunction sent a false signal to the control room and that signal met the design criteria to activate an automatic shutdown, she said.

Although the problem has been narrowed to the false signal, the plant will remain shut down while operators complete a thorough review of what caused the signal to malfunction, Lyon said.

The reactor restarted April 19 after being out of service for maintenance and refueling. Then on April 26, operators reduced the reactor's operating power based on indications of problems with the feedwater system. The plant had been operating at reduced levels while equipment repairs were in progress.